1. Field of the Invention
This invention relates generally to data processing systems and more particularly to data processing systems in which signals are transferred between memory units and a central processing unit and are stored in memory units in groups of predetermined size. The signal groups are generally referred to as words. In order to utilize memory units efficiently, related data signals called operands are stored consecutively in memory locations without regard to memory word boundaries. Thus, the boundaries of the memory word locations do not in general coincide with the boundaries of the operands and the transfer between memory units and the central processing unit of words including operand boundaries can contain unrelated signals. This invention relates to the problem of retaining the unrelated signals.
2. Description of the Prior Art
It is known in the prior art to include in a processing instruction involving manipulation of an operand by a central processing unit, the address of the operand in a memory unit. The instruction can include signals indicating that an operand resulting from the central processing unit manipulation is to be stored in the memory unit address in which the original operand was stored.
It will be clear that when an operand is transferred between the memory unit and the central processing unit, the first word and the last word will in general include data that is not part of the operand. The non-operand data, which can be referred to as rewrite data, can be disregarded during manipulation of the operand by the central processing unit.
However, when an operand resulting from a manipulation of the central processing unit is returned to the memory unit to be stored at the address of the operand involved in the operation, the rewrite data must be inserted in the operand boundary words. Otherwise, this data can be lost upon entry of the operand into the memory unit address.
It is known in the prior art, after manipulation of an operand by the central processing unit, to interrupt the operation of the data processing unit by retrieving the first and last words of the operand address. After the retrieval of these words, the rewrite data is added to the first and last words of the operand resulting from the central processing unit manipulation, and the resulting words which contain the operand are entered into memory. It is clear that the requirement to suspend the normal sequence to retrieve the rewrite data from the first and last words containing the operand can result in inefficient operation.
It is therefore an object of the present invention to provide an improved data processing unit.
It is a further object of the present invention to provide an improved method of adding non-operand data to a word containing both non-operand data and operand data, the operand data resulting from manipulation of a central processing unit.
It is yet another object of the present invention to rewrite non-operand data into the words containing the least and the most significant data of a resulting operand when the resulting operand replaces a prior operand in a memory address.
It is a more particular object of the present invention to store the first and the last words of an operand containing non-operand or rewrite data where the operand is to be replaced in the memory address after an identified operation by a resulting operand. The stored words are utilized to rewrite the non-operand information into the first and last words of a group of words, including the resulting operand, to be stored in the memory unit.
It is still another object of the present invention to provide that non-operand data in the beginning and ending words of a group of words including an operand to be manipulated by the central processing unit are inserted in the beginning and ending words of a group of words including an operand resulting from the manipulation and are to be inserted into a location previously occuppied by the original operand.
It is still a further object of the present invention to restore non-operand data to the first and last data words of an operand resulting from manipulation of a central processing unit without a second retrieval from memory of words containing non-operand information.